fixed render funciton and obsbuilder

marl_factory_grid/modules/batteries/entitites.py

@@ -49,9 +49,6 @@ class Battery(_Object):
         summary.update(dict(belongs_to=self._bound_entity.name, chargeLevel=self.charge_level))
         return summary
 
-    def render(self):
-        return None
-
 
 class Pod(Entity):
 

marl_factory_grid/modules/batteries/groups.py

@@ -23,6 +23,10 @@ class Batteries(Collection):
     def var_has_position(self):
         return False
 
+    @property
+    def var_can_be_bound(self):
+        return True
+
     @property
     def obs_tag(self):
         return self.__class__.__name__

marl_factory_grid/modules/doors/entitites.py

@@ -12,7 +12,7 @@ class DoorIndicator(Entity):
         return d.VALUE_ACCESS_INDICATOR
 
     def render(self):
-        return None
+        return []
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)

marl_factory_grid/modules/items/groups.py

@@ -135,7 +135,7 @@ class DropOffLocations(Collection):
 
     @staticmethod
     def trigger_drop_off_location_spawn(state, n_locations):
-        empty_positions = state.entities.empty_positions[:n_locations]
+        empty_positions = state.entities.empty_positions()[:n_locations]
         do_entites = state[i.DROP_OFF]
         drop_offs = [DropOffLocation(pos) for pos in empty_positions]
         do_entites.add_items(drop_offs)